1. Field
The disclosed embodiments belong to the field of structures including structural elements made out of composite material.
More particularly, the aspects of the disclosed embodiments are directed to producing a structural element made out of composite material, whose thickness must be obtained locally during the production of said element, while complying with precise tolerances.
2. Brief Description of Related Developments
In the working structures, that is to say those undergoing significant strains regarding the resistance of the materials used to produce said structures, the elements constituting the structure must be assembled with precision, which assumes that the dimensions of the different assembled elements are perfectly coordinated between themselves, at least with respect to assembly interfaces.
In order to achieve this, the different elements are defined by the design offices with relatively close tolerances, which the manufacturing workshops must comply with.
Such problems occur in a general manner in numerous mechanical industries, and today are well managed when the elements of the structure to be assembled are produced from metallic materials, for which the machining techniques enable the dimensions of the elements to be managed by means of suitable precautions and relatively high costs, even when the tolerances on the dimensions are close.
When the structural elements are made out of composite materials, a situation occurring more and more often, in particular in aeronautic construction due to the advantages of composite materials in terms of mass for example, it turns out to be much more difficult to comply with the dimensions set for the assembly operations due to the difficulties in complying with the dimensional tolerances in the production of such elements.
The composites entering into the production of the elements concerned by the disclosed embodiments are composites comprising long fibres, for example carbon, Kevlar® or glass, maintained in an organic matrix, generally a resin cured by polymerisation (thermosetting resin) or by cooling after hot working (thermoplastic resin).
Such elements are also likely to include other materials, in particular metallic materials such as inserts or plates, for example Glare® formed by alternating fibre plies impregnated with resin and thin metallic sheets.
Indeed, these long fibre composite materials are usually applied in successive plies, which can reach up to several hundred in number, for which the orientation of the fibres is chosen at the level of each ply in order to obtain the mechanical characteristics desired for the element.
Due to the superpositioning of numerous plies during the production of the element and the methods of applying successive plies, the tolerances on the thicknesses of each ply and each interply accumulate and, in the worst case scenario, the finished element has thicknesses which are locally, in an assembly area, above the maximum value provided for or lower than the minimum value provided for.
Solutions are therefore very limited.
If the element has a thickness higher than the maximum thickness acceptable, an excessive thickness, said element cannot be assembled as with another element which does meet the expected dimensions.
The thickness of the element cannot be reduced by removing material, as this removal of material would locally reduce the number of fibre plies, and therefore affect the structural resistance of the part.
It is theoretically possible to produce the other element to be assembled with the suitable dimensions for taking into account the excessive thickness, however such a solution is not really acceptable from an industrial point of view, except for small quantities produced or for elements with a high added value, and in addition, this solution affects the interchangeability of the elements and therefore creates additional difficulties in the event of required repairs.
The most industrial solution therefore consists in scrapping the part with unacceptable, excessive thicknesses and abandoning its use.
If the element has a thickness lower than the minimum thickness acceptable, the most generally applied solution consists in filling the intermediate space created by the lack in thickness by means of spacers and/or an assembly sealant.
This solution is however limited to certain thicknesses due to its impact on the structural resistance of the assembly, and can, in particular for aeronautical structures, require the intervention of a design office in order to accept the solution for each case study and derogate from the nominal definition of the element and its assembly.
In addition, these solutions create industrial problems which are difficult to accept for practical purposes:
the filling operations are delicate, long and vary from one assembly to another;
the filling operations cannot be efficiently automated;
handling of the filling sealants is delicate due to toxicity risks for humans and the environment.
There is therefore real interest in developing a solution which would enable the thickness of elements made out of composite materials to be managed in view of producing a structural assembly, which is exactly what is proposed by the disclosed embodiments.